This invention relates to electro-optical systems and methods of providing such devices whose optical properties in dependence on applied voltage can be adjusted. It can be used also to provide a memory of the applied voltages and/or for coupling electronic and optical parts of an apparatus.
The invention can be applied to integrated optical circuits, the optical transmission of information, the optical storage of data and images for example in computers and holograms, computer optical logic, opto-electronic and similar areas in which voltage-dependent optical devices are advantageously applicable.
Various devices are known in this field which enable a connection between electronic and optical elements of systems. These devices are made of crystalline solids and the changes in the optical properties result from relatively high voltages (over 15 V), e.g. by the linear electro-optical effect (Pockels-effect), which further requires polarized light, or these changes are obtained, e.g. by means of piezo-electrically generated sound waves, but here the rise time is limited by the time required to achieve stable oscillations. None of the known voltage-dependent optical devices has at the same time the function of a voltage-memory [see, e.g., A. B. Sharma, S. H. Halem, M. M. Butusov (Editor), "Optical Fiber Systems and their Components", page 126, Springer-Verlag Berlin (1981) or Ping-King Tien and Joseph A. Giordmaine, "Building the Integrated Optical Circuit", Optical Spectra, June 1981, page 50].
The present invention is based on the selective excitation (i.e. excitation in certain selected levels) of a solid in which are isolated photo-absorbing centers which may be impurity centers such as molecules, atoms and ions. When the photo-absorbing centers are resonantly excited (e.g. by a laser), the numbers of such centers can be reduced if the solid is kept at a temperature at which a process reverse to the process of this reduction cannot take place without external influence. Such a process of reduction, which is called "persistent spectral hole-burning" is described for the material of the preferred embodiment, comprising dye molecules in thin organic films, e.g. polymer films or Langmuir- Blodgett films, in literature (U. Bogner, Physical Review Letters, Vol. 37, 1976, page 909).
A special application of such reduction process for a "Frequency Selective Optical Data Storage System" is described in U.S. Pat. No. 4,101,976 issued July 18, 1978, in which FIG. 4 illustrates hole burning. GB Patent Specification No. 1,351,812 describes how the number of optical centers absorbing laser light can be changed by applying an electric field, but no optical voltage-memory or voltage-tuned optical memory is described.